Method and apparatus for treating dehydrogenation catalysts

ABSTRACT

A method and apparatus are disclosed for regenerating and/or stabilizing the activity of a dehydrogenation catalyst used in dehydrogenating an alkylaromatic hydrocarbon to obtain an alkenylaromatic hydrocarbon, the method comprising the steps of continuously or intermittently adding to a reactant stream an effective amount of an alkali metal compound without interrupting the dehydrogenation reaction.

This is a divisional of application Ser. No. 08/088,306 filed on Jul. 7,1993, now U.S. Pat. No. 5,461,179.

The present invention relates generally to a method and apparatus forgreatly extending the useful life of a catalyst bed used in thecatalytic dehydrogenation of alkylaromatic hydrocarbons whilemaintaining a very high level of conversion and a very high level ofselectivity and without the need to interrupt the conversion process.

BACKGROUND OF THE INVENTION

It is known in the art that an alkylaromatic hydrocarbon can becatalytically dehydrogenated to form an alkenylaromatic hydrocarbon,such as in the conversion of ethylbenzene to styrene. The prior artteaches a variety of different dehydrogenation catalysts and processparameters, each having different advantages and disadvantages. Ingeneral, the prior art teaches that certain tradeoffs ordinarily must bemade between level of conversion and level of selectivity, between levelof conversion and catalyst life, and so forth. For example, thedisadvantage of obtaining a higher degree of dehydrogenation of thealkylaromatic in some processes may be a lower level of selectivity,i.e., a higher percentage of undesired dehydrogenation byproducts.Obviously, it is most advantageous and cost-effective to obtain bothhigh levels of conversion and high levels of selectivity, if possible.

Catalyst life, and the related cost factors, is another importantprocess parameter in these dehydrogenation reactions. First are thecosts related to the catalyst itself. Although the unit cost of thecatalyst may not be great, because of the large amounts of catalystrequired as well as the cost of disposing of used, contaminated catalystin an environmentally acceptable way, the life of the catalyst and theability to regenerate used catalyst are critical elements in acommercial dehydrogenation process. Second are the costs related toshutting down a large, perhaps multistage, dehydrogenation reactor,operating at temperatures on the order of 600° C., in order to eitherreplace or regenerate the catalyst bed. In addition to the obvious laborcosts, there are also the capital costs of having expensive equipmentoff-line for any length of time. Heat losses add still further costs tothis catalyst replacement or regeneration step. Of even greatersignificance is the cost of lost production during the shutdown period.

Thus, on the one hand, it is preferred to maximize catalyst life. But,on the other hand, normal catalyst degeneration during use tends toreduce the level of conversion, the level of selectivity, or both,resulting in an undesirable loss of process efficiency. Various possibleexplanations for the typical degeneration of dehydrogenation catalystsduring use are found in the literature. These include carbonization ofcatalyst surfaces, physical breakdown of the interstitial structures ofthe catalysts, loss of catalization promoters, and others. Depending onthe catalyst and the various process parameters, one or more of thesemechanisms, or other mechanisms not yet identified, may be at work.

Although the prior art teaches various methods for regenerating usedcatalyst in order to restore temporarily and only partially thecatalyst's effectiveness, these methods generally involve stopping thedehydrogenation, shutting down the dehydrogenation reactor or, in somecases, removing the catalyst for external regeneration. Furthermore, theprocess impact of such periodic catalyst regeneration is an undesirablesaw-tooth pattern of output: levels of conversion and selectivity startout relatively high but slowly and continuously deteriorate until thepoint where the catalyst is regenerated to restore a relatively highlevel of conversion and selectivity. But, immediately thereafter,catalyst effectiveness begins again to deteriorate. As a result, it isnot possible utilizing conventional catalyst regeneration methods toachieve steady-state process conditions at high levels of conversion andselectivity.

For example, German Patentschrift Nos. DD 298 353, DD 298 354, DD 298355, DD 298 356, and DD 298 357 teach a 3-step process for regeneratingthe catalyst bed in an ethylbenzene-to-styrene dehydrogenationcomprising: (1) shutting down the reaction and substituting a steam feedfor the mixed steam-ethylbenzene feedstream; (2) followed by a heattreatment step; and (3) followed by introducing potassium ions in asteam feed (for example by vaporizing KOH or K₂ CO₃). None of thesepatents, however, teach or suggest in situ catalyst regeneration withoutprocess interruption. The process of these German patents would becostly, cumbersome, and result in the kind of undesirable saw-toothpattern mentioned above.

U.S. Pat. No. 4,551,571 (Sarumaru et al.) teaches a different approachfor extending the life of the catalyst bed in an ethylbenzene-to-styrenedehydrogenation comprising the method of employing two kinds ofpotassium-containing dehydrogenation catalysts arranged in a particularway within the catalyst bed. U.S. Pat. No. 4,590,324 (Satek) is broadlydirected to dehydrogenation of an alkylaromatic compound containing atleast two carbon atoms and at least one alkyl group (for example,ethylbenzene) to an alkenylaromatic (such as styrene) by contact with aparticular catalyst. Satek's preferred catalyst comprises copper on asupport of aluminum borate. At col. 6, lines 57-63, Satek teaches thatthe catalyst "can be treated or doped with an alkali metal or alkalineearth metal compound for use in the dehydrogenation." Such a one-timedoping step is taught as being particularly advantageous for conversionof ethylbenzene to styrene. At col. 6, line 64-col. 7, line 18, Satekalso teaches that the oxides, hydroxides and salts of potassium, amongothers, are suitable agents for doping the catalysts. Satek furtherteaches that aqueous solutions of the doping agent can be added tofeedstocks going to a reactor. However, Satek only suggests doping hisparticular catalyst (copper on aluminum borate) in the preceding manner.Comparable teachings appear in U.S. Pat. No. 4,645,753 (Zletz et at.),which is referred to as a copending patent application at col. 6, lines61-63 of Satek.

U.S. Pat. No. 4,902,845 (Kim et al.) teaches a process for extending thelife of an iron oxide containing catalyst in alkyl aromaticdehydrogenation processes consisting of adding oxygen or oxygenprecursors (such as peroxides) to the reactant feedstream for in situtreatment of the catalyst bed without interruption of thedehydrogenation process. But, the examples and test data presented inKim et al. demonstrate that this procedure is not really very effective.Indeed, FIG. 1 of Kim et al. suggests that the Kim et al. process atmost slows, but does not reverse, the degradation of the dehydrogenationcatalyst.

U.S. Pat. Nos. 4,451,686 (DeClippeleir et al.), 5,190,906 (Murakami etal.), 4,064,187 (Soderquist et al.), 4,277,369 (Courty et al.), and4,287,375 (Moller et al.), are all directed to various embellishments ofthe conventional ethylbenzene-to-styrene catalytic dehydrogenationprocess. DeClippeleir et al., for example, suggests one mechanism bywhich catalyst regeneration might occur in teaching that a small amountof an alkali metal oxide, particularly potassium oxide, "promotes theremoval of coke and tars by reaction with steam through the water-gasreaction, and mitigates therefore a carbon build-up on the catalystsurface," (col. 1, lines 39-43). U.S. Pat. No. 4,737,595 (Jones et al.)does not relate specifically to ethylbenzene-to-styrene conversion butdoes address catalyst regeneration in a method for broadlydehydrogenating dehydrogenatable hydrocarbons. The aforementioned U.S.patents are incorporated herein by reference.

None of the foregoing patents, however, discloses any method for eitherregenerating or stabilizing catalyst activity in order to maintainsubstantially steady-state dehydrogenation conditions, over extendedperiods of time and at very high levels of conversion and selectivitywithout process interruption. These and other problems with andlimitations of the prior art are overcome with the catalyst regeneratingand/or stabilizing method and apparatus of this invention.

OBJECTS OF THE INVENTION

Accordingly, it is a principal object of this invention to provide amethod of and apparatus for regenerating and/or stabilizing adehydrogenation catalyst.

It is also an object of this invention to provide a method of andapparatus for regenerating a dehydrogenation catalyst in situ.

Another, more specific object of this invention is to provide a methodof and apparatus for continuously or intermittently regenerating adehydrogenation catalyst without process interruption and so as tomaintain substantially steady-state reaction conditions at high levelsof conversion and selectivity over extended time periods.

Still a further object of this invention is to provide an improvedmethod for dehydrogenating ethylbenzene to styrene in the presence of acatalyst containing iron and one or more alkali metal compounds.

Specifically, it is an object of this invention to provide a method ofand apparatus for regenerating and/or stabilizing a dehydrogenationcatalyst containing iron and one or more alkali metal compounds byadding to the reactant feedstream an alkali metal compound.

Other objects of the invention will in part be obvious and will in partappear hereinafter. The invention accordingly comprises the methods,processes, and apparatus involving the several steps and the relationand order of one or more of such steps with respect to each of theothers and to the apparatus exemplified in the following detaileddisclosure, and the scope of the application of which will be indicatedin the claims.

SUMMARY OF THE INVENTION

The dehydrogenation catalyst regeneration and/or stabilization method ofthis invention comprises the steps of continuously or intermittentlyadding to a reactant feedstream an effective amount of an alkali metalcompound during continuation of the dehydrogenation process. The methodof this invention may also include the step of gradually increasing thereaction zone temperature. The method of this invention can be utilized,for example in the catalytic dehydrogenation of ethylbenzene to styrene,to achieve substantially steady-state reaction conditions at high levelsof conversion and selectivity.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of one embodiment of the method andapparatus of this invention.

FIG. 2 illustrates a preferred method and associated apparatus forcarrying out the alkali metal compound addition step of this invention.

FIG. 3 illustrates an alternative method and associated apparatus forcarrying out the alkali metal compound addition step of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method of this invention broadly comprises regenerating and/orstabilizing the activity of a dehydrogenation catalyst used in thecatalytic dehydrogenation of an alkylaromatic hydrocarbon to obtain aspecific desired alkenylaromatic hydrocarbon. Such dehydrogenationcatalysts are well known in the art and are commercially available. Ingeneral, such catalytic dehydrogenation processes are carried out attemperatures ranging from about 400° C. to about 700° C., and preferablybetween about 500° C. to 700° C., by contacting a preheated feedstream,containing a mixture of the alkylaromatic hydrocarbon and steam, with aparticular dehydrogenation catalyst. The process can be carried out insingle or multistage reactors having fixed catalyst beds or in fluidizedbeds. The choice of starting alkylaromatic hydrocarbon, dehydrogenationcatalyst, reaction temperature and proportion of alkylaromatichydrocarbon to steam in the feedstream will affect, in part, theresulting alkenylaromatic hydrocarbon as well as the efficiency andselectivity of the conversion process.

In particular, using the aforementioned process, ethylbenzene isconverted to styrene by contact with a dehydrogenation catalystcontaining iron and at least one alkali metal compound. For example, theethylbenzene-to-styrene conversion can be advantageously carried out atreaction temperatures ranging from about 500° C. to about 700° C.,preferably from about 550° C. to about 650° C., and at reactionpressures ranging from about 3 to about 20 psia, preferably from about 5to about 9 psia. The steam-to-hydrocarbon ratio in the reactantfeedstream may range from a ratio of about 0.6:1 to about 3:1 steam toethylbenzene by weight, preferably a ratio of about 1.0:1 to about 2.0:1steam to ethylbenzene by weight. The space velocity may range from about0.2 to about 1.2 kilograms of ethylbenzene per hour per kilogram ofcatalyst.

Beginning the ethylbenzene-to-styrene conversion process with freshcatalyst, following start-up there is typically an initial conditioningperiod lasting from about 3-45 days characterized by high initialactivity followed by rapid deactivation. For example, during the initialconditioning period, the overall level of conversion of ethylbenzene tostyrene might drop off to below about 55 mole % and the level of styreneselectivity might fall to below about 93 mole %. Thereafter inconventional ethylbenzene-to-styrene dehydrogenation processes, thelevel of catalyst activity continues to fall albeit at slower rates thanduring the initial conditioning period. In a multi-stage reactor, thelevel of conversion of ethylbenzene to styrene in each stage might dropby about one-third, for example, from about 30-36 mole % to about 20-24mole %, during the initial conditioning period, and then continue todrop at a slower rate thereafter. The end of the initial conditioningperiod in this process generally can be identified by those skilled inthe art as the point at which the slope of the line plotting conversionlevel over time flattens out. As noted previously, the prior art hassuggested a number of possible explanations for the gradualdeterioration of catalyst activity, but no single mechanism seems tofully account for this phenomenon.

Whatever the explanation, the continued process deterioration beyond theinitial conditioning period leads to a number of problems anddisadvantages. First, the efficiency of the conversion process isreduced. Unreacted ethylbenzene must be separated from the othercomponents of the output stream for recycling. Styrene must similarly beseparated from the unreacted ethylbenzene as well as from other reactionproducts. Second, instead of a relatively uniform output stream havingrelatively constant ratios of styrene, ethylbenzene and miscellaneousby-products, process deterioration leads to an output stream ofever-changing composition. Third, at some point, the level of conversionor the level of styrene selectivity or both fall sufficiently low thatthe process is no longer economically viable. At this point, the processwould have to be shut down and the catalyst either replaced orregenerated by conventional means.

One technique for maintaining the conversion level of ethylbenzene tostyrene is to raise the reaction temperature. This can be accomplished,for example, by increasing the temperature of the reactant stream or byadding heat to the reactor chamber. The reaction temperature can beincreased slowly and substantially continuously or it can be increasedperiodically in increments. The impact of such reaction temperatureincreases is to increase the rate of reaction in order to offset thecontinuing deterioration of catalyst activity. But, there are relativelynarrow limits to the utility of this temperature-raising technique. Inparticular, above a certain temperature, the mechanical temperaturelimit of the catalyst or the equipment is approached. Beyond this point,further temperature increases lead to degradation of the catalyst'sphysical structure and degradation of equipment integrity. As theaforedescribed limit is approached, therefore, the process would have tobe shut down and the catalyst either replaced or regenerated byconventional means. Although this temperature-raising technique extendscatalyst life somewhat and can be utilized (for example, throughcontinuous or small, frequent reaction temperature increases) tomaintain relatively constant conversion of ethylbenzene for a limitedperiod of time, by itself it is of limited utility for the reasonsstated.

By contrast, the method of this invention is capable of restoring and/orstabilizing the activity of the catalyst and consequently extending thelife of the catalyst far beyond what can be achieved with conventionalprocesses. More particularly, the method of this invention is capable ofrestoring the activity of the dehydrogenation catalyst to substantiallythe same high levels of conversion and selectivity as those establishedat the end of the initial conditioning period. The method of thisinvention is also capable of stabilizing the activity of the catalyst atthose same high levels of conversion and selectivity for extendedperiods of time beyond those achievable with conventional processes. Themethod of this invention may also be utilized in tandem with theaforedescribed temperature-raising technique for additional benefits offurther increasing the catalyst life or increasing the ethylbenzeneconversion. The method of this invention broadly comprises the steps ofcontinuously or intermittently adding to a reactant feedstream of analkylaromatic hydrocarbon an effective amount of an alkali metalcompound sufficient to regenerate, stabilize, or enhance the activity ofthe dehydrogenation catalyst and thereby to restore and to maintain highlevels of conversion and selectivity. The term "maintain" as used hereinis intended to be construed to mean--to keep in a state of repair,efficiency or validity; to preserve from failure or decline over aprotracted period of time, e.g. for many months or years. The method isof particular utility in connection with regenerating and/or stabilizinga dehydrogenation catalyst containing iron and at least one alkali metalcompound. Such dehydrogenation catalysts are well known in the art andsome of those that are available commercially include: the S6-20, S6-21and S6-30 series from BASF Corporation; the C-105, C-015, C-025 andC-035 series from Criterion Catalyst Company, L.P.; and the G-64 andG-84 series (including catalyst G-84C used in Examples 1-4 hereinafter)from United Catalysts, Inc. These catalysts typically contain 40-80% Fe₂O₃, 5-30% K₂ O, and other promoters. All of such and similar catalystsare considered within the scope of this invention.

The method of this invention can be used in connection with thecatalytic dehydrogenation of virtually any alkylaromatic hydrocarbon toa corresponding alkenylaromatic hydrocarbon. The appropriate combinationof alkylaromatic hydrocarbon, catalyst and reaction conditions in orderto obtain a particular desired alkenylaromatic hydrocarbon is generallywell known in the art and, in any event, would be a matter of choice androutine experimentation. The method of this invention is of particularutility in connection with regenerating and/or stabilizing adehydrogenation catalyst in the process of converting ethylbenzene tostyrene.

Dehydrogenation of ethylbenzene to styrene over an iron oxide catalystsystem provides a useful illustration of the method of this invention.Dehydrogenation of ethylbenzene to styrene typically is accomplishedcommercially with an iron oxide catalyst promoted with potassiumcompounds in the presence of excess superheated steam.

Catalysts useful in this invention are those containing an oxide ofiron. Preferably, a substantial portion of such iron oxide is in theform Fe₃ O₄, although Fe₂ O₃ may be reduced in situ by hydrogen to Fe₃O₄. Usually, further reduction to FeO leads to an inactive catalystspecies. Other materials can be present in minor amounts as promoters orstabilizers. Examples of such added materials are nonoxidation catalyticcompounds of Groups IA, IB IIA, IIB, IIIA, VB, VIB, VIIB and VIII andrare earths, such as zinc oxide, magnesium oxide, chromium or coppersalts, potassium oxide, potassium carbonate, oxides of chromium,manganese, aluminum, vanadium, magnesium, thorium and molybdenum. Forexample, an iron oxide catalyst useful in this invention may containabout 50 to about 95 wt. % iron red as Fe₂ O₃, about 5 to about 30 wt. %potassium compound, measured as potassium oxide, such as potassiumcarbonate and potassium oxide and up to about 20 wt. % of othercompounds, measured as their oxides, such as compounds of vanadium,cadmium, magnesium, manganese, nickel, rare earths, chromium, andmixtures thereof. Preferable iron oxide-containing catalysts containabout 70 to about 90 wt. % iron oxide (as Fe₂ O₃) about 5 to about 30wt. % potassium compound (as K₂ O) and up to about 20 wt. % othercompounds measured as their oxides. One specific example of an ironoxide-containing catalyst suitable for ethylbenzene dehydrogenationcontains about 80-90 wt. % iron oxide (as Fe₂ O₃), about 8-15 wt. %potassium oxide, about 1-3 wt. % chromium oxide and about 0-1 wt. %vanadium oxide.

Compounds which catalyze oxidation of hydrocarbons, such as platinum orpalladium salts, should be substantially absent from ironoxide-containing catalysts used in this invention.

Various iron oxide-containing catalysts and processes using suchcatalysts have been reported widely. Examples of such catalysts andprocesses include U.S. Pat. Nos. 2,111,726; 2,408,140; 2,414,585;2,426,829; 2,461,147; 2,870,228; 2,945,960; 3,084,125; 3,179,706;3,179,707; 3,205,179; 3,291,756; 3,306,942; 3,361,683; 3,387,053;3,424,808; 3,703,593; 3,849,339; 3,907,416; 4,039,601; 4,143,083;4,144,197; and 4,152,300, all incorporated by reference herein.Commercially, suitable iron oxide catalysts are sold under trademarkssuch as Shell-105, Shell-115, Shell-015, UCI-G64D, UCI-G64E, UCI-G64Fand UCI-G64I.

Conversion processes using iron oxide-containing catalysts includehydrocarbon dehydrogenation such as ethylbenzene to styrene,ethyltoluene to vinyltoluene, cumene to alpha-methylstyrene and butenesto butadiene. Other conversion processes are formation of gasolinefraction hydrocarbons from synthesis gas, dealkylaration ofalkylaromatics such as toluene to benzene, and synthesis of ammonia fromnitrogen and hydrogen. Broadly, conversion processes using ironoxide-containing catalysts are run at temperatures ranging from about150° to about 1000° C. at pressure of about 0.01 to about 100atmospheres (1-10,000 kPa) at liquid hourly space velocities of about0.01 to about 10 hr⁻¹.

FIG. 1 is a flowchart that schematically illustrates one embodiment ofthe method of this invention wherein an alkali metal compound is addedto an incoming feedstream as well as to the partially converted reactantstream passing between the stages of a multi-stage reactor. Although forpurposes of discussing FIG. 1, the terms "feedstream" and "partiallyconverted reactant stream" are used to help identify particular stagesin the conversion process, elsewhere in this description these terms areconsidered generic and interchangeable. In FIG. 1, incoming feedstream 1may be the feed of alkylaromatic hydrocarbon, for example ethylbenzene,and incoming feedstream 2 may be steam. As shown in FIG. 1, an effectiveamount of an alkali metal compound is added continuously orintermittently to feedstream 2 by alkali metal supply means 46.Alternatively, the alkali metal compound can be added to feedstream 1.Feedstreams 1 and 2, including any alkali metal compound, are combinedinto reactant stream 3 and directed to the inlet of reactor 50, which isloaded with a suitable dehydrogenation catalyst. Alternatively, thealkali metal compound can be added after feedstreams 1 and 2 have beencombined into reactant stream 3 prior to reactor 50. Partial conversionof the alkylaromatic hydrocarbon, for example ethylbenzene to styrene,occurs in reactor 50.

The partially converted reactant stream or exit stream 4 emerging fromreactor 50 is then passed through a reheater 52 to restore the heat lostin reactor 50 and to reestablish the optimum reaction temperature.Additional alkali metal compound from alkali metal supply means 66 isadded continuously or intermittently to the partially converted reactantstream 5 emerging from the reheater 52 before reactant stream 5 isdirected to the inlet of reactor 54. Alternatively, additional alkalimetal compound can be added to the partially converted reactant stream 4coming from reactor 50 before it enters the reheater. Reactor 54 is alsoloaded with a suitable dehydrogenation catalyst. Further conversion ofthe alkylaromatic hydrocarbon occurs in reactor 54. It will be apparentto those skilled in the art that additional downstream reactor stages,such as a third or a fourth stage, each loaded with a suitable catalyst,may be utilized to obtain still further conversion of the alkylaromatichydrocarbon. Addition of alkali metal compound to the reactant stream inaccordance with the method of this invention may be advantageouslyemployed between some or all of the reactor stages in a multi-stagereactor.

As shown in FIG. 1, the apparatus of this invention may advantageouslyinclude monitoring means for monitoring the chemical composition of theexit streams coming from the outlets of any one or more of the reactorstages, such as monitoring means 42 and 62 associated respectively withreactant streams 4 and 6. The monitoring means may also advantageouslybe coupled to activating means, such as electrical wires 44 and 64respectively, for signalling and activating alkali metal compound supplymeans, such as pump or injection means 46 and 66 respectively, locatedupstream from the subject reactor stage, namely reactor 50 and 54respectively.

The monitoring means can be adapted through conventional technology tosend a signal to the alkali metal compound supply means whenever theexit stream from the subject reactor stage falls below a predeterminedlevel of conversion or selectivity, which indicates degradation ofcatalyst activity in the reactor stage. Upon activation by the signalfrom the associated monitoring means, the alkali metal compound supplymeans would begin to supply alkali metal compound at a predeterminedrate to the associated feedstream or reactant stream. For example, onsignal from monitoring means 42 that the conversion or selectivity inreactor 50 had dropped below a certain level, pump means 46 would beginto supply alkali metal compound to feedstream 2. The apparatus can bedesigned on activation to continue supplying alkali metal compound for apredetermined period of time or until another signal from the associatedmonitoring means signals that catalyst activity in the subject reactorstage has been restored to the desired activity level.

As an alternative to the automated, intermittent addition systemdescribed above, it is also within the scope of this invention tocontinuously add alkali metal compound to the respective feedstreams orreactant streams. This may be combined with continuous or intermittentmonitoring of one or more reactor stage exit streams. Upon signs ofcatalyst degradation in any reactor stage, means for increasing theaddition of alkali metal compound to a feedstream or reactant streamupstream from the subject reactor stage can be manually activated. Theincreased rate of addition of alkali metal compound can be for a limitedperiod of time until the desired catalyst activity level is restored orelse it can be maintained at the new, higher rate.

FIG. 2 schematically illustrates in somewhat greater detail onepreferred method and associated apparatus for adding alkali metalcompound to a feedstream or a reactant stream in accordance with thisinvention. Conduit 10 in FIG. 2 contains and directs stream 12 in thedirection of a reactor stage containing dehydrogenation catalyst asillustrated by the arrows. Stream 12 may represent, for example,feedstreams 1 or 2, the combined reactant stream 3, or partiallyconverted reactant streams 4 and 5 as shown in FIG. 1. Alkali metalcompound is added continuously or intermittently to stream 12 in theform of aqueous solution 22 delivered through injection means 24 at theoutlet end of an injection tube 20 fitted through an aperture in thewall of conduit 10. Stream 14, downstream from the outlet end ofinjection tube 20, represents a feedstream or a reactant stream that hasbeen mixed with alkali metal compound in accordance with this invention.

FIG. 3 schematically illustrates an alternative method and associatedapparatus for adding alkali metal compound to a feedstream or a reactantstream in accordance with this invention. Conduit 10 in FIG. 3 containsand directs stream 12 in the direction of a reactor stage containingdehydrogenation catalyst as illustrated by the arrows 12 and 14. Stream12 may represent, for example, feedstreams 1 or 2, the combined reactantstream 3, or partially converted reactant streams 4 and 5 as shown inFIG. 1. Conduit 10 further defines an adjacent vessel 30 in open vaporcommunication with the flow path of stream 12 and capable of retainingsolid or liquid matter. Alkali metal compound in a solid or liquid stateis fed as necessary to the interior 34 of vessel 30, through feed 32, soas to gradually vaporize and diffuse into the passing stream, asindicated by the arrows inside vessel 30. Stream 14, downstream fromvessel 30, represents a feedstream or a reactant stream that has beenmixed with alkali metal compound in accordance with this invention.

The alkali metal compounds that are useful in carrying out the method ofthis invention include all non-halogen sources of alkali metal ions. Asused in connection with this invention, the term "alkali metal" is meantto include, but without limitation thereto, potassium, sodium, lithiumand other less-common members of the group IA metals of the periodictable, such as rubidium and cesium. Cost considerations will ordinarilydictate the choice of a potassium or sodium compound, however. For someapplications, members of the group IIA metals of the periodic table(e.g. magnesium, calcium, and so forth) may also have utility. Selectionof an appropriate alkali metal compound is considered a matter ofroutine experimentation. In connection with the dehydrogenation ofethylbenzene to styrene, the preferred alkali metal compounds arepotassium compounds, more particularly one or more compounds selectedfrom the group consisting of potassium oxide, potassium hydroxide, andpotassium carbonate. It is also within the scope of this invention touse mixtures of two or more alkali metal compounds. Because halogenions, such as chloride, have typically been found to poison thedehydrogenation catalyst, alkali metal compounds such as potassiumchloride should generally be avoided.

The amount of alkali metal compound to be added to a feedstream orreactant stream in accordance with this invention may vary dependingupon the catalyst, the alkylaromatic hydrocarbon, the reactionconditions, and the alkali metal compound itself. An effective amount oran effective rate of addition of alkali metal compound to the reactantstream sufficient to maintain high levels of conversion and selectivitycan be determined by routine experimentation in order to optimize systemperformance. In general, it has been found that an effective amount ofthe alkali metal compound comprises from about 0.01 to about 100 partsby weight of alkali metal compound per million parts of the reactantstream, preferably from about 0.10 to about 10 parts by weight, onaverage over a representative time frame. A representative time frame asused herein means that period of time during which high levels ofconversion and selectivity are maintained without further addition ofalkali metal compound. It is also within the scope of this invention tovary the amount of alkali metal compound in the reactant stream overtime to restore or to maintain optimum system performance.

The alkali metal compound may be added to the reactant stream eithercontinuously or intermittently and, if intermittently, at regular orirregular intervals. Where the alkali metal compound is addedintermittently, the amounts added and the intervals selected should besuch to insure addition of an effective amount sufficient to restore orto maintain high levels of conversion and selectivity. In general, suchamounts will comprise from about 0.01 to about 100 parts weight ofalkali metal compound per million parts of the reactant stream onaverage over a representative time frame.

The addition of the alkali metal compound to the reactant stream may beaccomplished in a number of ways. One such addition method is to add thealkali metal compound in dry, solid, powdered form to a reactant stream.Alternatively, a solid lump of or a vessel containing the alkali metalcompound can be placed in the path of the heated reactant stream andallowed to gradually vaporize into the passing stream. Another andparticularly preferred addition method is to add the alkali metalcompound in the form of an aqueous solution into a reactant stream, forexample as described above in connection with FIG. 2. Because of ease ofhandling and the ability to automate the process, as described inconnection with FIG. 1, the addition of alkali metal compound in theform of an aqueous solution will probably normally be the preferredcommercial application of this invention. Still another addition methodis to inject the alkali metal compound in a liquid form into thereactant stream. Still another addition method is to pre-vaporize thealkali metal compound and inject the vapor into the reactant stream.

The method of this invention will be better understood by reference tothe following examples and test data, which are illustrative only andare not limiting of the scope or practice of this invention.

EXAMPLE 1

Steam and ethylbenzene at a molar ratio of 12 to 1 and a rate of 825grams/hr was introduced into a reactor constructed with one inchschedule 40 stainless steel pipe and was heated in an 8-zone electricaloven. A total of 390 grams of a catalyst designated as G-84C andmanufactured by United Catalysts, Inc., was loaded in the reactor infour sections. The reactor was packed with inert alumina balls on top ofthe first catalyst zone, between catalyst zones, and below the fourthzone. The reaction mixture entered the reactor at a temperature of 250°C. and was preheated to 598° C. as it entered into the first catalystsection. The average temperature of the four catalytic sections was keptwithin 1° C. of 594° C. The reactor outlet was maintained at atmosphericpressure. Deactivation of catalyst was observed between 854 and 1298hours on-stream time as shown in Table 1 below. The styrene selectivityincreased slightly during this period due to decline in ethylbenzeneconversion. At 1310 hours, the test unit was shut down and a stainlesssteel container loaded with 5.4 grams of ACS grade KOH was placed on topof the alumina. After the reactor was restarted, a small amount of KOHvaporized continuously and was brought into contact with catalyst by thereactor feed stream. The temperature at the container was controlledsuch that KOH vapor pressure was equivalent to about 5 ppm (by weight)of total feed. The activity and selectivity of the catalyst were foundto improve continuously between 1310 and 1500 hours. The catalystactivity then remained stable at about 2.3 percentage points higher inconversion and about 0.3 percentage points higher in styrene selectivitythan before addition of KOH. At 1642 hours, the ethylbenzene conversionwith KOH addition was 4.4 percentage points higher than what it wouldhave been at the same on-stream time had the catalyst deactivated at thesame rate of KOH. The introduction of KOH. The styrene selectivity withKOH addition was 0.4 percentage points higher than that without KOH whencompared with data at comparable ethylbenzene conversion (e.g., that at854 hours).

                                      TABLE 1                                     __________________________________________________________________________             On-Stream Time (hours)                                                        854                                                                              874                                                                              1283                                                                             1298                                                                             1476                                                                             1506                                                                             1535                                                                             1563                                                                             1620                                                                             1642                                      __________________________________________________________________________    KOH (ppm)                                                                              0  0  0  0  5  5  5  5  5  5                                         Overall Conversion                                                                     61.7                                                                             61.7                                                                             59.3                                                                             59.2                                                                             60.9                                                                             61.5                                                                             61.4                                                                             61.5                                                                             61.5                                                                             61.6                                      (mole %)                                                                      Overall Selectivity                                                                    96.6                                                                             96.7                                                                             96.9                                                                             96.7                                                                             96.9                                                                             97.0                                                                             97.0                                                                             97.0                                                                             97.1                                                                             97.1                                      (mole %)                                                                      __________________________________________________________________________

EXAMPLE 2

A second reactor constructed and loaded similar to that described inexample 1 was loaded with 32.5 grams of G-84C catalyst in the top zone.The reactor was operated at average temperatures between 594° and 612°C., steam-to-ethylbenzene ratio between 9 and 12, at an outlet pressureof 14.7 psia between 0 and 3341 hours. The ethylbenzene feed ratethroughout the run was kept the same as that in example 1. Deactivationof the top zone catalyst was observed between 1722 and 1888 hours, asshown in Table 2 below, while the zone was kept within 1° C. of 597° C.The steam-to-ethylbenzene ratio was 12 molar. At 3341 hours, the unitwas shut down and a superheater, which allowed loading of KOH into itlater on without interrupting the reaction, was installed upstream ofthe reactor inlet. After the unit was restarted, the top zone catalystcontinued to deactivate while it was kept within 1° C. of 622° C. (Table2). The steam-to-ethylbenzene ratio was 9 and the reactor outletpressure was 14.7 psia. At 3481 hours, 7.0 grams of ACS grade KOH wasloaded into the superheater while the reaction was uninterrupted. Thesuperheater temperature was controlled to allow small amounts of KOH tovaporize continuously and be brought into contact with catalyst by thereaction mixture. The vapor pressure of KOH at such temperature wasequivalent to about 9 ppm (by weight) of total feed. The ethylbenzeneconversion in the top zone improved rapidly from 5.1% to 12.3% in 103hours while the zone was kept within 1° C. of 622° C., then increasedgradually to 12.9% in 176 hours, and remained at such high level for 630hours. This example illustrates that the catalyst which is immediatelyexposed to the feedstream is most susceptible to deactivation and alsoreceived the greatest benefit from the method of this invention.

                                      TABLE 2                                     __________________________________________________________________________              On-stream Time (hours)                                                        1722                                                                             1800                                                                             1888                                                                             3347                                                                             3418                                                                             3473                                                                             3504                                                                             3584                                                                             3760                                                                             4132                                                                             4390                                  __________________________________________________________________________    KOH (ppm) 0  0  0  0  0  0  9  9  9  9  9                                     Top Zone Conversion                                                                     9.4                                                                              8.9                                                                              8.3                                                                              6.0                                                                              5.6                                                                              5.1                                                                              6.9                                                                              12.3                                                                             12.9                                                                             12.8                                                                             12.9                                  (mole %)                                                                      __________________________________________________________________________

EXAMPLE 3

The reactor described in example 2 was run until the KOH in thesuperheater was depleted. The outlet pressure and thesteam-to-ethylbenzene ratio were then adjusted to 6 psia and 8:1 molar,respectively. The top zone temperature was kept within 1° C. of 622° C.Ageing of the top zone catalyst was observed between 4731 and 5022 hourswhile the styrene selectivity was deteriorating. At 5022 hours, anadditional 1.90 grams of KOH was loaded into the superheater and thesuperheater temperature was controlled to give a KOH vapor pressureequivalent to about 2 ppm (weight) of total feed. The top zoneconversion went up from 9.2% to 11.4% in 24 hours, then stabilized atabout 11.0% for an extended period of time while the top zone styreneselectivity improved from 94.8 to 96.8%, as shown in Table 3 below.

                                      TABLE 3                                     __________________________________________________________________________              On-stream Time (hours)                                                        4731                                                                             4925                                                                             5022                                                                             5026                                                                             5034                                                                             5046                                                                             5062                                                                             5114                                                                             5159                                                                             5204                                     __________________________________________________________________________    KOH (ppm) 0  0  0  2  2  2  2  2  2  2                                        Top Zone Conversion                                                                     11.0                                                                             9.6                                                                              9.2                                                                              10.1                                                                             11.1                                                                             11.4                                                                             11.0                                                                             11.1                                                                             11.1                                                                             11.0                                     (mole %)                                                                      Top Zone Selectivity                                                                    95.0                                                                             94.8                                                                             94.8                                                                             94.5                                                                             95.1                                                                             96.4                                                                             96.7                                                                             96.9                                                                             96.8                                                                             96.7                                     (mole %)                                                                      __________________________________________________________________________

EXAMPLE 4

In example 3, the average temperature of the four catalyst zones waskept within 1° C. of 613° C. The reactor outlet pressure was 6 psia, andthe steam-to-ethylbenzene ratio was 8:1 molar. The overall conversiondecreased from 70.4% to 69.3% between 4728 and 5019 hours before thesecond batch of KOH was loaded. The overall styrene selectivity wasstable at 96.9%. After KOH was loaded into the superheater at 5022hours, the overall conversion increased steadily from 69.3 to 70.4% intwo days and remained above that high level subsequently. The styreneselectivity remained stable at 96.9% in this period, i.e., the sameselectivity was observed at a higher conversion.

                                      TABLE 4                                     __________________________________________________________________________             On-stream Time (hours)                                                        4728                                                                             4860                                                                             4932                                                                             5019                                                                             5031                                                                             5047                                                                             5071                                                                             5121                                                                             5168                                                                             5211                                      __________________________________________________________________________    KOH (ppm)                                                                              0  0  0  0  2  2  2  2  2  2                                         Overall Conversion                                                                     70.4                                                                             70.1                                                                             69.7                                                                             69.3                                                                             69.8                                                                             70.2                                                                             70.4                                                                             70.4                                                                             70.5                                                                             70.5                                      (mole %)                                                                      Overall Selectivity                                                                    96.9                                                                             96.9                                                                             96.9                                                                             96.9                                                                             96.8                                                                             96.9                                                                             96.9                                                                             96.9                                                                             96.9                                                                             97.0                                      (mole %)                                                                      __________________________________________________________________________

The foregoing examples illustrate that the method of this invention iseffective in restoring the activity of a partially deactivated catalystand in stabilizing the conversion of ethylbenzene to styrene at a highlevel, while simultaneously maintaining or improving the selectivity tostyrene.

Since certain changes may be made in the above-described apparatuses andprocesses without departing from the scope of the invention hereininvolved, it is intended that all matter contained in the abovedescription shall be interpreted in an illustrative and not in alimiting sense.

Having described the invention, what we claim is:
 1. Method ofregenerating and stabilizing the activity of a dehydrogenation catalystconsisting essentially of iron oxide catalyst promoted with alkali metalwhile continuing the catalytic dehydrogenation of an alkylaromatichydrocarbon to obtain an alkenylaromatic hydrocarbon comprising thesteps of:(a) forming a mixed reactant stream consisting essentially ofsaid alkylaromatic hydrocarbon, steam, and an effective amount of analkali metal compound equivalent to a continuous addition of about 0.01to about 100 parts per million by weight of alkali metal compoundrelative to the weight of the total alkylaromatic hydrocarbon and steam,said effective amount of alkali metal compound being sufficient tomaintain substantially constant levels of catalyst activity; and (b)bringing said mixed reactant stream into contact with saiddehydrogenation catalyst, while continuing the catalytic reaction beyondan initial catalyst conditioning period.
 2. Method of claim 1 whereinsaid catalyst consists essentially of about 40-80% Fe₂ O₃ and about5-30% K₂ O.
 3. Method of claim 1 wherein said alkali metal compound isadded continuously to said reactant stream.
 4. Method of claim 1 whereinsaid alkali metal compound is added intermittently to said reactantstream.
 5. Method of claim 1 wherein said alkylaromatic hydrocarbon isethylbenzene and said alkenylaromatic hydrocarbon is styrene.
 6. Methodof claim 1, wherein said alkali metal compound is a potassium or sodiumcompound.
 7. Method of claim 6 wherein said sodium or potassium compoundis selected from the group consisting of sodium hydroxide, potassiumhydroxide, sodium oxide, potassium oxide, sodium carbonate, potassiumcarbonate, and mixtures thereof.
 8. Method of claim 1 wherein saidalkali metal compound is potassium hydroxide.
 9. Method of claim 1wherein said alkali metal compound is added in dry, solid form to saidreactant stream.
 10. Method of claim 9 wherein a vessel containing saidalkali metal compound is placed in the flow path of said reactantstream.
 11. Method of claim 1 wherein said alkali metal compound isadded in the form of an aqueous solution to said reactant stream. 12.Method of claim 1 wherein said alkali metal compound is added to saidreactant stream as a vapor.
 13. Method of claim 1 wherein said alkalimetal compound is added to said reactant stream as a liquid.
 14. Methodof claim 1 wherein said catalytic dehydrogenation is carried out atsubstantially constant reaction temperature.
 15. Method of claim 1further comprising the step of gradually increasing the reactiontemperature.
 16. Method of claim 1 wherein said alkylaromatichydrocarbon is a polyalkylated monoaromatic compound.
 17. Method ofclaim 16 wherein said polyalkylated monoaromatic compound isethyltoluene.
 18. Method of claim 16 wherein said polyalkylatedmonoaromatic compound is diethylbenzene.
 19. Method of claim 16 wheretosaid polyalkylated monoaromatic compound is methylethylbenzene. 20.Method of claim 16 wherein said polyalkylated monoaromatic compound isethylxylene.
 21. Method of claim 16 wherein said polyalkylatedmonoaromatic compound is ethyltrimethylbenzene.
 22. Method of claim 1wherein said alkylaromatic hydrocarbon is an alkylated biphenylcompound.
 23. Method of claim 22 wherein said alkylated biphenylcompound is ethylbiphenyl.
 24. Method of claim 22 wherein said alkylatedbiphenyl compound is ethyldimethylbiphenyl.
 25. Method of claim 1wherein said alkylaromatic hydrocarbon is an alkylated naphthalenecompound.
 26. Method of claim 25 wherein said alkylated naphthalenecompound is ethylnaphthalene.
 27. A method of regenerating andstabilizing the activity of a dehydrogenation catalyst, consistingessentially of iron oxide catalyst promoted with alkali metal, used inthe catalytic dehydrogenation of an alkylaromatic hydrocarbon in aseries of reactors containing said catalyst to obtain an alkenylaromatichydrocarbon, said method comprising the steps of: adding to a feedstreamconsisting essentially of said alkylaromatic hydrocarbon reactionproduct from said reactors and steam an effective amount of an alkalimetal compound equivalent to a continuous addition of about 0.01 toabout 100 parts per million by weight of alkali metal compound relativeto the weight of said feedstream, said effective amount being sufficientto maintain substantially constant levels of catalyst conversion andselectivity, to form a mixed reactant stream; and, directing said mixedstream into said reactor while continuing said catalytic conversionbeyond an initial catalyst conditioning period of about 3-45 days. 28.Method of claim 27 wherein said catalyst consists essentially of about40-80% Fe₂ O₃ and about 5-30% K₂ O.
 29. Method of claim 27 wherein saidalkali metal compound is added continuously to said feedstream. 30.Method of claim 27 wherein said alkali metal compound is addedintermittently to said feedstream.
 31. Method of claim 27 wherein saidalkylaromatic hydrocarbon is ethylbenzene and said alkenylaromatichydrocarbon is styrene.
 32. Method of claim 27 wherein saidalkylaromatic hydrocarbon is a polyalkylated monoaromatic compound. 33.Method of claim 32 wherein said polyalkylated monoaromatic compound isethyltoluene.
 34. Method of claim 32 wherein said polyalkylatedmonoaromatic compound is diethylbenzene.
 35. Method of claim 32 whereinsaid polyalkylated monoaromatic compound is methylethylbenzene. 36.Method of claim 32 wherein said polyalkylated monoaromatic compound isethylxylene.
 37. Method of claim 32 wherein said polyalkylatedmonoaromatic compound is ethyltrimethylbenzene.
 38. Method of claim 27wherein said alkylaromatic hydrocarbon is an alkylated biphenylcompound.
 39. Method of claim 38 wherein said alkylated biphenylcompound is ethylbiphenyl.
 40. Method of claim 38 wherein said alkylatedbiphenyl compound is ethyldimethylbiphenyl.
 41. Method of claim 27wherein said alkylaromatic hydrocarbon is an alkylated naphthalenecompound.
 42. Method of claim 41 wherein said alkylated naphthalenecompound is ethylnaphthalene.
 43. Method of claim 27 wherein said alkalimetal compound is a potassium or sodium compound.
 44. Method of claim 43wherein said sodium or potassium compound is selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium oxide,potassium oxide, sodium carbonate, potassium carbonate, and mixturesthereof.
 45. Method of claim 27 wherein said alkali metal compound ispotassium hydroxide.
 46. Method of claim 27 wherein said alkali metalcompound is added in dry, solid form.
 47. Method of claim 27 whereinsaid alkali metal compound in a solid or liquid state is placed in theflow path of said feedstream.
 48. Method of claim 27 wherein said alkalimetal compound is added in the form of an aqueous solution.
 49. Methodof claim 27 wherein said alkali metal compound is added as a vapor. 50.Method of claim 27 wherein said alkali metal compound is added as aliquid.
 51. Method of claim 27 wherein said catalytic dehydrogenation iscarried out at substantially constant reaction temperature.
 52. Methodof claim 27 further comprising the step of periodically increasing thereaction temperature.
 53. A method of regenerating and stabilizing theactivity of a catalyst consisting essentially of iron oxide catalystpromoted with alkali metal while continuing the catalytic conversion ofsynthesis gas to gasoline fraction hydrocarbons comprising the steps of:(a) forming a mixed reactant stream consisting essentially of saidsynthesis gas and an effective amount of an alkali metal compoundequivalent to a continuous addition of about 0.01 to about 100 parts permillion by weight of alkali metal compound relative to the weight ofsaid reactant stream, said effective amount being sufficient to maintainhigh levels of conversion: and (b) bringing said mixed reactant streaminto contact with said catalyst while continuing said catalyticconversion beyond an initial catalyst conditioning period.
 54. A methodof regenerating and stabilizing the activity of a catalyst consistingessentially of iron oxide catalyst promoted with alkali metal whilecontinuing the catalytic dealkylation of an alkylaromatic hydrocarbon toobtain an aromatic hydrocarbon comprising the steps of:(a) forming amixed reactant stream consisting essentially of said alkylaromatichydrocarbon and an effective amount of an alkali metal compoundequivalent to a continuous addition of about 0.01 to about 100 parts permillion by weight of alkali metal compound relative to the weight ofsaid reactant stream, said effective amount being sufficient to maintainhigh levels of conversion; and (b) bringing said mixed reactant streaminto contact with said catalyst while continuing said catalyticconversion to said aromatic hydrocarbon beyond an initial catalystconditioning period.
 55. Method of claim 54 wherein said alkylaromatichydrocarbon is toluene and said aromatic hydrocarbon is benzene.
 56. Amethod of regenerating and stabilizing the activity of a catalystconsisting essentially of iron oxide catalyst promoted with alkali metalwhile continuing the catalytic synthesis of ammonia from nitrogen andhydrogen comprising the steps of:(a) forming a mixed reactant streamconsisting essentially of said nitrogen and hydrogen and an effectiveamount of an alkali metal compound equivalent to a continuous additionof about 0.01 to about 100 parts per million of weight of alkali metalcompound relative to the weight of said reactant stream, said effectiveamount being sufficient to maintain high levels of conversion: and (b)bringing said mixed reactant stream into contact with said catalystwhile continuing the catalytic conversion to ammonia beyond an initialconditioning period.